1. Field of Invention
The field of the currently claimed embodiments of this invention relates to compositions of matter and articles of manufacture that use the compositions, and more particularly to compositional variations of tungsten tetraboride and articles of manufacture that use the compositional variations of tungsten boride.
2. Discussion of Related Art
In many manufacturing processes, materials must be cut, formed, or drilled and their surfaces protected with wear-resistant coatings. Diamond has traditionally been the material of choice for these applications, due to its superior mechanical properties, e.g. hardness>70 GPa (1, 2). However, diamond is rare in nature and difficult to synthesize artificially due to the need for a combination of high temperature and high pressure conditions. Industrial applications of diamond are thus generally limited by cost. Moreover, diamond is not a good option for high-speed cutting of ferrous alloys due to its graphitization on the material's surface and formation of brittle carbides, which leads to poor cutting performance (3). Other hard or superhard (hardness≥40 GPa) substitutes for diamond include compounds of light elements such as cubic boron nitride (4) and BC2N (5) or transition metals combined with light elements such as WC (6), HfN (7) and TiN (8). Although the compounds of the first group (C, B or N) possess high hardness, their synthesis requires high pressure and high temperature and is thus non-trivial (9, 10). On the other hand, most of the compounds of the second group (transition metal-light elements) are not superhard although their synthesis is more straightforward.
To overcome the shortcomings of diamond and its substitutes, we have been pursuing the synthesis of dense transition metal borides, which combine high hardness with synthetic conditions that do not require high pressure (11, 12). For example, arc melting and metathesis reactions have been used to synthesize the transition metal diborides OsB2 (13, 14), RuB2 (15) and ReB2 (16-20). Among these, rhenium diboride (ReB2) with a hardness of ˜48 GPa under a load of 0.49 N has proven to be the hardest (16, 21). The boron atoms are needed to build the strong covalent metal-boron and boron-boron bonds that are responsible for the high hardness of these materials (12). Because of this, it is expected that by increasing the concentration of boron in these types of lattices, the hardness could increase. Most transition metals, however, form compounds with low boron content. Tungsten is one of the few transition metals that is known for its ability to form higher boron content borides. In addition to tungsten diboride (WB2), which is not superhard (22, 23), tungsten is able to form tungsten tetraboride (WB4), the highest boride of tungsten that exists under equilibrium conditions (24-26). Advantages of this material over other borides are: i) both tungsten and boron are relatively inexpensive, ii) the lower metal content in the higher borides reduces the overall cost of production since the more costly transition metal is being replaced by less expensive boron thus reducing the cost per unit volume and iii) the higher boron content lowers the overall density of the structure, which could be beneficial in applications where lighter weight is an asset.
Tungsten tetraboride was originally synthesized in 11966 (24) and its structure assigned to a hexagonal lattice (space group: P63/mmc). The possibility of high hardness in this material was first suggested by Brazhkin et al. (27) and we discussed its potential applications as a superhard material in a Science Perspective in 2005 (12). Recently, Gu et al. (28) reported hardness values of 46 and 31.8 GPa under applied loads of 0.49 and 4.9 N, respectively, and a bulk modulus of 200-304 GPa without giving any synthetic details or even presenting an X-ray diffraction pattern. Since superhard materials have shown a large load-dependant hardness (13, 16), commonly referred to as the “indentation size effect”, reporting a single hardness value for these materials is insufficient and suggests that a more detailed study is needed. Therefore, here we examine the hardness of tungsten tetraboride using micro- and nano-indentation. Furthermore, with a valence electron density of 0.485 e− Å−3 (11), which is comparable to that of ReB2 (0.477 e− Å−3) the bulk modulus of 200-304 GPa reported by Gu et al. for this material seems low compared to other superhard transition metal borides such as ReB2, with a bulk modulus of 360 GPa (16), and therefore requires further investigation. Since the purity of superhard materials directly influences their mechanical properties (29), the existence of other borides of tungsten in the samples might explain the anomalously low bulk modulus. Making solid ingots of phase pure WB4 is especially challenging since the tungsten-boron phase diagram indicates that WB2 is thermodynamically favorable with any W:B molar ratio below 1:12 (24). There thus remains a need for improved hard materials and articles that use the improved materials.